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LED Testing 

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LED Testing 

Written By: Steven Robert Cypherd 


Breadboard kit (1) 
Digital multimeter (1) 


LEDs are everywhere and in every project you see anywhere. They show that your 
microprocessor unit is alive and running the program you wrote. LEDs are tough little lights 
that light up everything including you. They provide instant gratification. Here is my simple 
tester. There are lots of kits to show you electronics and soldering and a lot of them use 
LEDs. Even high-end electronics use LEDs that nobody ever sees to tell a technician that a 
circuit is working. 

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LED Testing 

Step 1 — LED Testing 

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LED Testing 

Testin an LED 



DC Volts 

Power Current 

C 3 Jf™ lts S tep 2 or 1&2 

Step 3 

Resistors: 220, 330, 
460, 820, 1000 

Disconnect Pot for 
Ohms test. 

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• Build the circuit as shown in the 
picture. Always wire the pot so that 
clockwise rotation increases the 
voltage or lessens the resistance. 

• Wiring connections: Ground to 
LED-; Pot-2 to LED+; Pot-3 to 
Volts+; DMM to DC Volts; DMM- 
to ground; DMM+ to LED+. 

• Turn the pot counter-clockwise all 
of the way. Connect power. 

• LEDs are Current Devices and they 
have three states. 1 : Lit but dim. 
Low light voltage. Saves batteries. 
2: Lit and steady. Normal 
brightness. 3: Too bright. You are 
over-driving them and they will 
burn out sooner. Lastly, state 4: 
Too bright and dimming. You have 
just killed an LED. 

• Rotate the pot clockwise until the 
LED begins to light. You can note 
this as the lowest voltage that will 
light the LED. Note the voltage on 
the DVM. Keep the voltage to less 
than 2.2 volts for most LEDs. 

• Keep watching the LED as you are 
turning the pot clockwise until the 
LED brightness is steady. This is 
your LED's operating voltage. Turn 
the pot counter-clockwise to the 
lowest point where the brightness 
of the LED is steady. This voltage 
will make the LED last longer. 

• Turn the pot too far and the LED 
will start to dim. Just turn the pot 

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LED Testing 

counter-clockwise until the LED 
brightness is steady. Keep the 
voltage at about 2 volts for 
maximum brightness for most 
• Some LEDs have a circuit in them 
that controls voltage and current to 
the LED. They will not light until 
they have reached their normal 
operating voltage. They come on 
and stay at their normal brightness 
over a wide range of voltage and 
current. Usually this is between 4 
and 12 volts. 

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• Batteries can produce a lot of 
current, so test LEDs as shown 
and use the largest resistor you 
can for the best brightness. Normal 
LEDs run about 7 - 30- milli- 
amperes (mA) each. Some high- 
brightness LEDs and LEDs with 
chips in them can draw 200 mA 
each. Above 5 volts I usually use a 
460 - 1 000 (1 K) ohm resistor for 
normal LEDs and 22 - 200 ohms 
for high-brightness LEDs or LEDs 
with chips. The idea is to protect 
your LEDs for a longer life. 

• Limiting the current through each 
LED with a resistor on the positive 
lead will give your projects a longer 
life. It does not save your batteries. 
Lowering the brightness of your 
LEDs can save your batteries a 

• Current testing: Testing the current 
drawn by the LED is a better test 
for a current-driven LED. Replace 
the red wire from Pot-2 to LED+ 
with a second DMM or use the 
same DMM as shown. 

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• Run Step 1 and note how the 
voltage changes slowly, but when 
you hit the operating voltage of the 
LED the current will jump up to the 
normal current of the LED. As you 
increase the current the brightness 
of the LED will start to dim. Turn 
the pot counter-clockwise until the 
current and the brightness is 
steady. Always keep the current on 
the low side and your LEDs will last 

• Repeat for more LEDs. 

• Finding the resistor for the LED at 
that voltage: 

• Disconnect Power 

• Set DMM to Ohms. Disconnect the 

• Measure across Pot-2 and Pot-3 to 
get the resistor you will need for 
that LED at that voltage. Find the 
closest higher resistor for your 
LED. Keeping the current lower will 
keep the LED healthy. Remember, 
the brighter the LED the shorter its 
life will be. 

• Reconnect the pot. 

• Turn the pot full counter-clockwise. 
Repeat the above steps for more 

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LED Testing 

LEDs light the world up and they are easy and fun. 

This document was last generated on 2012-11-03 12:30:46 AM. 

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